Hydraulic circuits are commonly employed for activating clutch and brake mechanisms on vehicles especially agricultural and industrial tractors. A specific function for such a hydraulic circuit is in the operation of a power take-off shaft located on the rear of agricultural tractors. Normally, a clutch is employed having multiple drive plates and driven plates. The plates are axially engaged by an annular piston which is operated by hydraulic pressure from a low pressure hydraulic source. In tractors, the hydraulic source is commonly a gear pump which supplies a plurality of low pressure functions, some directly and others, including the clutch actuator, by way of a pressure regulating valve. This pump also supplies the transmission lubrication system. As is common, the pump has to be of substantial capacity to be sure of supplying all functions without loss of adequate pressure. In addition, the pump has to be capable of supplying fluid to the hydraulic functions at a substantial rate in order to fill the hydraulic lines rapidly and in order to take up the free movement in the annular piston in the actuator. Free movement arises from normal tolerances and also because the plates of the clutch are spring-biased to separate completely each time the clutch is disengaged. Such separation ensures adequate cooling of the plates by means of ventilation. If the low pressure pump does not supply sufficient capacity, there is a noticeable and unacceptable delay in engaging the clutch.
Once the plates of the clutch have come into initial engagement, they require only a very small further movement of the annular piston in order to build up full engaging pressure. Therefore, a pressure rise occurs very rapidly. Consequently, the clutch tends to snatch into rapid engagement thereby producing torque shocks which may be potentially dangerous and damaging to the power take-off shaft and/or to the driven implement in the case of a tractor driven PTO.
In order to prevent such snatching of the clutch, attempts have been made to insert an automatic modulating valve upstream of the annular piston so as to control the buildup of pressure in a smooth manner. However, the tolerances of such valves are very critical and experience has shown that it is difficult to ensure reliable operation, especially as clutch wear takes place over the life of the vehicle. Now a hydraulic circuit has been invented which uses a pressure-responsive throttle valve to overcome the snatching problem present in hydraulic actuators.